This invention relates generally to a method for molding articles of synthetic resin powder material and more particularly to those having covered or encapsulated therein a core of uneven surface configuration including a depression and/or a protrusion or a curved area thereon.
Recently, in chemical and other industries, with the use of increasingly high pressures and temperatures in various fluid-handling systems, there is arising a demand for improvements in pressure- and heat-resisting properties of materials for component parts of such systems. Also, due to increase in the variety of chemicals used and liquids handled, improvements in corrosion resistance of the component parts are keenly needed. One known measure taken to improve the corrosion resistance of components of fluid-handling systems is to cover the surface of the system components, ordinarily made of metal material, with a lining uniformly formed thereon of an appropriate highly corrosion-resistant material. As materials for such lining, tetrafluoroethylene and other synthetic resins are in wide use and, among others, tetrafluoroethylene resin is suitable as a lining material for use in protecting various component units such as liquid control valves. It has been very difficult, however, to apply such lining material to component parts of any complicated exterior configuration such as that of the valve disc of a butterfly valve on account of the structural and manufacturing limitations involved, which result in more or less uneven flow of the tetrafluoroethylene or other synthetic resin powder used. For example, in the manufacture of the valve disc of a butterfly valve and particularly in the process of compression-molding the tetrafluoroethylene resin powder, the powder loaded in the mold tends to flow unevenly and be compacted to nonuniform density, particularly in its regions directly subjected to the molding pressure to form the opposite face regions of the valve member. Because of this, it has been usual that, in the heating stage following such compression molding, minute cracks are formed in the molded product or the latter is left in an unstable state liable to crack. To cope with this situation, it has been necessary to make a substantial allowance for removal of the cracked or unstable surface regions by an appropriate machining operation. This has not only caused loss of material but resulted in a substantial rise in the manufacturing cost on account of the machining and/or other finishing operation required. As an alternative measure to overcome the difficulties, an isostatic method of applying uniform pressure of liquid has been attempted. This method is effective to prevent occurrence of cracks or local embrittlements in the molded product resulting from any nonuniformity in density of the powder material, but it has not been fully put into practice with any success on account of its disadvantageous features, including extraordinarily large equipment cost, need for machining operation on the molded product, and the inability to obtain any satisfactorily high accuracy in the surface configuration of the product.